The present invention relates, generally, to an exposure apparatus, and, more particularly, to an exposure apparatus that exposes a plate, such as a wafer for a semiconductor device, and a glass plate for a liquid crystal display (“LCD”). The present invention is suitable, for example, for a so-called immersion exposure apparatus that immerses, in a liquid, a space between a plate and a final surface (of a final optical element) of a projection optical system, and exposes the plate via the liquid.
A conventional projection exposure apparatus uses a projection optical system to expose a circuit pattern of a reticle (or a mask) onto a wafer, etc., and a demand for a high-resolution exposure apparatus has recently increasingly grown. Immersion lithography is one attractive measure to meet this high-resolution demand. Immersion lithography promotes an increase of a numerical aperture (“NA”) of the projection optical system by replacing a medium at the wafer side of the projection optical system with a liquid. The projection exposure apparatus has an NA of n·sin θ, where n is a refractive index of the medium. The NA increases up to n when the medium has a refractive index greater than that of air, i.e., n>1. Immersion lithography intends to reduce the resolution R (which is defined as R=k1(λ/NA)) of the exposure apparatus, where k1 is a process constant, and λ is a wavelength of a light source.
When the exposure light is irradiated onto the liquid and the temperature, the chemical structure, and thus, the refractive index of the liquid change with time, the imaging performance of the projection optical system (exposure apparatus) deteriorates due to a focus error, and various aberrations, such as a spherical aberration and a curvature of field. The exposure apparatus should precisely transfer a reticle pattern onto the plate, and the transferred pattern is sensitive to the aberrations with a progress of the finer processing to the semiconductor device. In particular, applications of an aqueous solution and an organic solvent to the liquid are now under investigation to advance the high NA scheme of the projection optical system. It is foreseeable that the imaging performance of the exposure apparatus depends upon the change in the refractive index in the aqueous solution and organic solvent caused by temperature and chemical structure changes.
One proposed immersion exposure apparatus includes measuring means for measuring the refractive index of the liquid. See, for example, Japanese Patent Applications, Publication Nos. (“JPs”) 10-340864 and 2004-301825.
The device in JP 10-340864 measures an aberration of the projection optical system, calculates an amount of the change in the refractive index from the measurement result, and adjusts a refractive index by controlling an additive amount of an additive or a liquid component ratio. Since it takes an amount of time to measure an aberration, the device in JP 10-340846 cannot promptly adjust the change in the refractive index once the change occurs. Thus, it is not always preferable to use the aberration measuring unit of the projection optical system to correct the change in the refractive index of the liquid. In addition, a relationship between the aberration and the refractive index is not so clear. The device in JP 2004-301825 projects plural detecting (non-exposure) lights having different wavelengths and incident angles to the substrate, and calculates an amount of the change in the refractive index based on differences from the error amount of the surface position information of each detecting light. Thus, the device in JP 2004-301825 cannot precisely calculate an amount of the refractive index if the refractive index of the liquid changes due to two reasons, i.e., a temperature change and a chemical-structure change. In addition, the detecting light uses the non-exposure light, and the device in this document cannot directly measure the refractive index characteristic and the transmission characteristic of the exposure wavelength.